A large proportion of the world's population do not have access to a consistent and reliable supply of mains electricity. In some countries, particularly underdeveloped countries, the supply of electricity is rationed by means of “load shedding”, the creation of intentional power outages known as “rolling blackouts”. In many African, South Asian and Latin American countries, for example, rolling blackouts are a common occurrence and while they are often scheduled at fixed times of the day and week, allowing people to work around the known interruption times, they frequently occur unexpectedly and without warning.
Even in developed countries, the demand for electricity occasionally exceeds the power supply capability of the network, thereby requiring localised or widespread load shedding. In addition, failure of the mains electricity supply can occur due to unforeseen events such as natural disasters or terrorist attacks.
The operation of electrical equipment intended for permanent operation in areas subject to unreliable electricity supply can therefore be difficult. The loss of electrical power to lighting systems, telecommunications systems or refrigeration systems which, in underdeveloped countries, are frequently used to store medicines such as vaccines requiring storage within a strict limited range of temperatures, can be disastrous.
The problem is often mitigated to some degree by the provision of back-up batteries, for example lead acid batteries, to supply electricity to the electrical equipment in the event of a mains supply failure. In a typical telecommunications application, a plurality of 2V batteries or battery cells are provided in series to form a battery or battery pack developing a combined output voltage of 24V, 48V or 96V, depending on power requirements.
The performance of such batteries, and in particular their useable lifetime, is dependent to a great extent on their temperature. The Arrhenius equation defines the relationship between temperature and the rate at which a chemical reaction, for example the reaction within a battery, proceeds. It shows that the reaction rate increases exponentially as temperature rises. Since the lifetime of the battery is inversely proportional to the reaction rate, it follows that the lifetime of the battery decays exponentially with temperature rise.
In general, the reaction rate doubles (and thus the lifetime halves) for every 10° C. increase in battery temperature. Thus, a battery operating at 35° C. will have a useful lifetime around half that of a battery operating at 25° C. and around one quarter that of a battery whose temperature is maintained at 15° C.
Apart from the gradual deterioration of the battery over time, temperature effects can occasionally lead to premature failure of the cell. This can happen even under normal operating conditions if the rate of heat generated in the battery exceeds the rate of heat loss to the environment. In this situation the battery temperature will continue to rise leading to a condition known as “thermal runaway” which can have serious physical as well as operational consequences.
It is unfortunate that those regions of the world which are subject to unreliable mains electrical power and thus require battery back up for essential electrical systems are often those in which ambient temperatures are continuously high and in which the usable life of such batteries is therefore reduced. Battery cooling systems, such as air conditioning equipment, reduce the effect of high ambient temperatures on battery life. However, such devices usually require electrical power themselves and thus are uneconomical where mains power is not readily available.
It is against this background that the present invention has been conceived. The applicants have recognised that there is a need for an apparatus and/or method for reliably cooling batteries without requiring a constant source of electrical power.
The present applicants have previously proposed a novel form of refrigeration apparatus adapted to cool items without requiring a continuous power supply. This apparatus, which is the subject of co-pending patent application no. PCT/GB2010/051129, permits a refrigerated storage space to be maintained within a temperature range of 4-8° C. for up to 30 days following a loss of electrical power.
The applicants have recognised that the technical principles employed in this apparatus may be adapted for use in an apparatus or method for cooling batteries. Embodiments of the invention may therefore provide an apparatus or a method for cooling one or more batteries for extended periods of time without a continuous external supply of electrical power. Other aims and advantages of the invention will become apparent from the following description, claims and drawings.